In the doc premiering February 16, the 31-year-old singer
explains, "About two years ago, I was pregnant for the
first time. And I heard the heartbeat, which was the most
beautiful music I ever heard in my life."

Beyoncé continues, "I envisioned what my child would
look like ... I was feeling very maternal."

Beyoncé, for her part, has never spoken about the
miscarriage ... until now.

But the Super Bowl halftime singer dealt with her tragedy
by going back into the recording studio.

"I went into the studio and
wrote the saddest song I've ever written in my life," she says,
"And it was actually the
first song I wrote for my album. It was the best form of therapy for me,
because it was the saddest thing I've ever been through."

But soon after, Beyoncé and
Jay-Z learned another child was on the way.

Beyoncé eventually did
become pregnant with Blue Ivy and reveals her baby bump in the
doc.HBO

When discussing her daughter Blue Ivy, the new mom says, "Being
pregnant was very much like falling in love. You are so open. You are so
overjoyed. There's
no words that can express having a baby growing inside of you, so
of course you want to scream it out and tell everyone."

And despite being originally reluctant to let Beyoncé co-direct
her own documentary, HBO programming president, Michael Lombardo,
recently told GQ:

"You feel ... that vulnerability of
someone searching for the comfort place where they're willing to
be more than the image that has been fed to us—I was incredibly
moved by that ... I dare anybody to tune in and turn it
off."